1. Field of the Invention
The present invention relates to an optical transmission apparatus and method for restraining influence of wavelength dispersion characteristics or nonlinear effect of a transmission path. More specifically, the present invention relates to an optical transmission apparatus and method which adjusts rise time and/or fall time of signal light to be transmitted, to thereby reduce waveform deterioration of the signal light after transmission.
2. Description of the Related Art
Optical communication systems using fiber optical transmission lines are being used to transmit relatively large amounts of information. However, as users require larger amounts of information to be rapidly transmitted, and as more users are connected to the systems, a further increase in the transmission capacity of optical communication systems is required.
Therefore, there is a continual effort in increase transmission capacity of optical communication systems. In the electrical field of art, Time-Division Multiplexing (TDM) is being used to increase transmission capacity. In the optical field of art, Optical Time-Division Multiplexing (OTDM) and Wavelength-Division Multiplexing (WDM) are being used to increase transmission capacity. Further, optical fiber amplifiers, such as erbium doped fiber amplifiers (EDFA), are being used to realize very long distance optical transmission.
With high-speed transmission in a very long distance optical communication system of large-capacity which utilizes WDM and optical amplifiers, the waveform of the transmitted signal is distorted. This distortion is due, for example, to interaction of spectral spread of signal lights caused by modulation, and to phase chirping caused by a nonlinear effect of an optical fiber, with the wavelength dispersion characteristics of the transmission path. Distortion results in deterioration of transmission characteristics, thereby restricting the transmission rate and the transmission distance.
Therefore, in very long distance optical communications systems having large-capacity, it is important to restrain the wavelength dispersion characteristics of the transmission path.
A conventional technique for restraining influence of wavelength dispersion is disclosed in Japanese Unexamined Patent Publication Nos. 2-30233, 8-237222, 9-116493, and 5-183511, which are incorporated herein by reference. These Japanese Unexamined Patent Publications disclose several techniques, such as (a) a method for offsetting wavelength dispersion characteristics of a transmission path by providing a dispersion compensator which generates dispersion having a sign opposite to the dispersion in the transmission path, (b) a method in which a signal light is provided with pre-chirping at a transmitter, and (c) a method for restricting an influence of wavelength dispersion by combining the above-mentioned dispersion compensator with pre-chirping. In this respect, the term “pre-chirping” indicates that a baseband signal is, in addition to light intensity modulation therefor, intentionally provided with optical phase modulation or optical frequency modulation to restrain spread of waveform due to wavelength dispersion.
However, in these techniques, the amount of dispersion compensation and/or pre-chirping will vary depending on various parameters such as the type of transmission system, transmission rate, type of transmission path, and distance of transmission path. Thus, such amounts must be respectively optimized for each optical communication system. Amounts such as for dispersion compensation and/or pre-chirping, which are optimized for each optical communication system, are assumed to be fixed at the time of introduction of the system. Consequently, it is difficult to extensively change or fine-tune the amounts.
Additionally, it is difficult to commonly use a transmitter, to which pre-chirping is applied, among different types of optical communication systems, since the amount of pre-chirping is different for different optical communication systems. For example, even if it is attempted to commonly apply a transmitter in an optical communication system having a transmission rate of 10 Gb/s to an optical communication system having a transmission rate of 2.5 Gb/s, it is difficult to realize such an attempt because the optimum amount of pre-chirping would be different for each system.
As should be understood from the above, even for those optical communication systems which have configurations similar to one another, it is necessary to develop optical devices which are matched with each system, thereby increasing the number of parts to be developed, and thereby increasing cost.
Further, in a WDM optical communication system in which a plurality of signal lights having different wavelengths are multiplexed together and then transmitted, there occurs fluctuation of transmission qualities between channels (i.e., between signal lights multiplexed together). Such fluctuation is due, for example, to influence of dispersion slope of a transmission path. To avoid this fluctuation, it is necessary, for example, to adopt a dispersion compensator which is capable of compensating the dispersion slope, or to optimize a compensation amount for dispersion and/or an amount of pre-chirping, individually to each of the wavelengths. Unfortunately, such techniques are too expensive since the number of parts is increased if optimization is effected for each of the wavelengths. As a result, the cost of an optical communication system is greatly increased.